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Enemies spawn frequently and attack in large waves from across the map, and the player is intended to dodge their projectiles and charges by jumping and strafing rather than by taking cover. Health and armour are non-regenerative, and instead are topped up with the health and armour packs which are littered throughout the game. Available health is represented by a percentage number on the screen. The combat is high-paced and frenetic: The levels typically consist of a series of elaborate arenas, in which the player will defeat a set number of re-spawning enemies before proceeding to the next. Enemies range from soldiers with rocket launchers to gigantic charging robots to shrieking suicide bombers without heads and holding bombs. The environments include ancient Egyptian pyramids, tropical rainforests, and medieval castles. Serious Sam features cooperative gameplay through a split-screen mode and via the internet.

Serious Sam consists of two episodes: Serious Sam: The First Encounter and Serious Sam: The Second Encounter. A fan-made level-pack, Dark Island, was included in Serious Sam: Gold. The games are on separate CDs (in the UK version) and are identical to their individual releases, except that the Sam character model for The First Encounter was modified to match the one used in The Second Encounter. After the release of the remakes, both classic titles were added "Classic" to the name to prevent confusion.[1][2]

The First Encounter was the first game in the series; it had been in the making since 1996 and had a video showing its early game play in 2000. The First Encounter was originally developed by Croteam as a demonstrator for their engine and this is the reason why some countries saw its initial release priced at less than half the value of other games in the genre.

The game takes place after the events of Serious Sam 3: BFE and the introduction reveals what happened before the actual story took place. In ancient times, Earth was involved in a massive conflict between Mental, an evil extraterrestrial being who wants to rule the universe, and the Sirians, a technologically advanced sentient alien race that left many of its artifacts to be discovered by humanity. In the 22nd century, Mental and his army of grotesquely, twisted monsters return to Earth intending to destroy humanity. The humans are quickly overwhelmed and driven back to Earth. As a last resort, humanity uses the "Time-Lock". This mysterious Sirian artifact contains the power to transport a single person back to a chosen point in time. Because of his bravery in fighting these monsters, Sam "Serious" Stone was chosen to use the Time-Lock in hopes that he will defeat Mental and change the course of history. As the game opens, Sam appears in Egypt. He travels from one Egyptian monument to the next, fighting against the monsters Mental sent after him along the way. During Sam's journey, Sam is able to find and activate a hidden Sirian communicator in the shape of an obelisk located in Luxor, which summons a Sirian starship to Earth from deep space. Sam rushes to the Great Pyramids of Giza in order to rendezvous with the starship, only to be cornered by Mental's general, Ugh Zan III. After defeating the giant, Sam then teleports himself onto the starship. While Sam is aboard the "SSS Centerprice", he leaves a phone message to Mental telling him that "he has a special delivery package for him" and sets a course to Sirius, the Sirian homeworld.

The Second Encounter starts where the previous game left off, with Sam traveling to Sirius on the SSS Centerprice. Unfortunately, the starship is accidentally hit by the "Croteam crate-bus" and plummets down to Earth's surface. As the starship falls, Sam reads the coordinates and frets about crash-landing into Egypt again, but instead he crashes into Central America in the Mayan age, with the starship now heavily damaged upon impact. However, not all hope is lost, since the Sirians left a "back-up starship" on Earth, which was a fail-safe in case if anything ever happens to the SSS Centerprice. However, since this back-up starship is located in a different time and age than where and when Sam is currently in, he will have to uncover the locations of the Sirian portals that will help him reach his destination. With this new objective, Sam storms through Mesoamerica, then travels to Mesopotamia and finally to Medieval Europe where the back-up starship is located. During his journey, Sam battles two of Mental's portal guardians: a powerful, Mayan spirit named Kukulkan the Wind God and a biomechanical creature called the Exotech Larva. Finally, in front of the Cathedral of Sacred Blood, Sam confronts the last obstacle to overcome on his path to the Holy Grail, Mordekai the Summoner. After a glorious battle with Mordekai and his spawning minions, The Summoner is finally silenced forever. In the Cathedral, Sam lifts the Holy Grail in the palms of his hands, followed by his sarcastic confessions of his sins in a booth to Mental, during which he tells Mental that "he's coming to get him." During the end credits, Sam activates the back-up starship, which turns out to be a rocket, and travels back into space toward Sirius, with the "Croteam Big-Heads" cheering him on. The story continues on to Serious Sam II.

Other than the more varied locations, The Second Encounter features some additional weapons—a chainsaw, a flamethrower, a sniper rifle, and the powerful Serious Bomb—and a number of new monsters that Sam has to face. This game has also seen the release of a level-pack titled Dark Island, featuring fan-made levels for single and multiplayer mode. The Second Encounter soundtrack featured three instrumental songs from the Croatian heavy-metal band Undercode.

Croteam created their own engine for use in both The First Encounter and The Second Encounter. Named the "Serious Engine", it is designed to cope with extremely large view distances and massive numbers of models, unlike most FPS engines which are developed for a limited draw distance and only a few animating models (i.e. enemies) on screen at a time. The Serious Engine is very efficient, capable of maintaining dozens of moving enemies (often stampedes) and enormous enemies, even on a modest system challenging the well known id Tech, Unreal Engine or Source engines. The "Serious Engine" can render through both Direct3D or OpenGL and, while it does not support pixel or vertex shaders, it is optimised for Direct3D 7's hardware transformation, clipping and lighting. The "Serious Engine" is available for licensing from Croteam.[3]

A more powerful iteration of the "Serious Engine" was developed for use in Serious Sam 2 and is known as "Serious Engine 2". It supports many features of modern GPUsx such as pixel and vertex shaders, HDR, bloom and parallax mapping.

Serious Engine 3 was used in Serious Sam HD: The First Encounter and Serious Sam HD: The Second Encounter. It includes detailed shading, and enemies are completely remodeled to look more realistic. This engine is also being developed to harness the full capacity of HDR and High Definition mapping. An updated version, Serious Engine 3.5, is used in Serious Sam 3: BFE.

In November 2001, Global Star Software announced the completion of a Palm OS version of The First Encounter developed by InterActive Vision, to be released in early December. The game included 15 levels, and came in both color and monochrome.[4] The game was released on December 6, 2001, as well as two demos for the color and the monochrome versions of the game.

The Xbox edition of Serious Sam had notable differences including the model of Sam being changed from the model of The First Encounter to form a more cartoon style Sam, removing the shades to make him look less like Duke Nukem and more like he does today. The Xbox version includes all of the levels from both of the PC games. As a result, weapons from The Second Encounter were added to the levels that were part of The First Encounter on PC. The levels were made smaller by removing the areas outside of the playing area. Unlike the PC version, Sam can't exit too far out of the playing area. This is due to the smaller amount of memory available in the Xbox. The gameplay was given a more arcade touch. A life system was implemented into the single-player game making use of the score. Every 100,000 points grants the player a new life, which allows respawning at the place of death rather than at a save point. Combo points were added for killing multiple enemies at the same time, which allows the player to increase his/her score at a fast pace. Further changes to the game include the addition of treasure and the tweaking of save points. Save points are designated by a red phone booth. Auto-aiming was also added to the Xbox version to compensate for the degree of control usually offered by a keyboard and mouse setup.

Both episodes were remade using Serious Engine 3. They featured updated visuals, better performance and lush environments, all in high-definition. The remakes featured 16-player online co-op (4-player in the Xbox Live Arcade version), deathmatch and a new game-mode called "Co-Op Turnament".[5][6]

The remake of the first episode, Serious Sam HD: The First Encounter, was released on November 24, 2009 for Microsoft Windows through Steam and on January 13, 2010 for Xbox 360 through the Xbox Live Arcade. The remake of the second episode, Serious Sam HD: The Second Encounter was released on April 28, 2010 for Microsoft Windows through Steam and on September 22, 2010 for Xbox 360 through the Xbox Live Arcade. On January 14, 2011, the DLC called "Fusion" was released on Steam, which merged the levels of The First Encounter with The Second Encounter. This DLC was free for everyone who owned both encounter on Steam. Additionally, The Second Encounter HD was expaned with new piece of DLC called "Legend of the Beast". Released on May 15, 2012, it contained new maps for the Survival and Multiplayer modes and a new, short set campaign missions.[7]

The First Encounter was the recipient of various editorial and consumer praise, with an overall 83.60% review ratio on Game Rankings,[8] and 87% on Metacritic,[9] as of June 2015. It also received numerous awards, including multiple Game of the Year award.